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Alkylidyne Complexes of Molybdenum and Tungsten That Contain the [(3,4,5-C6H2F3NCH2CH2)2NMe]2- Ligand Frank V. Cochran and Richard R. Schrock* Department of Chemistry, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139 Received January 5, 2001 Summary: Reduction of (C6F5NHCH2CH2)2NMe with LiAlH4 produces (3,4,5-C6H2F3NHCH2CH2)2NMe (H2[F3NMe]) in good yield. Reactions between H2[F3NMe] and MCl4 (M ) Mo, W) in the presence of NEt3 yield pseudooctahedral paramagnetic compounds of the type [Et3NH]{[F3NMe]MCl3}. Treatment of [Et3NH]{[F3NMe]MoCl3} with 3 equiv of Me3CCH2MgCl produced fivecoordinate [F3NMe]Mo(CH2CMe3)(CCMe3), while treatment with 3 equiv of Me3SiCH2MgCl produced fivecoordinate paramagnetic [F3NMe]Mo(CH2SiMe3)2. Upon heating, [F3NMe]Mo(CH2SiMe3)2 is converted into diamagnetic {[F3NMe]Mo(CSiMe3)}2. Reactions between [Et3NH]{[F3NMe]WCl3} and Me3CCH2MgCl or Me3SiCH2MgCl lead to five-coordinate diamagnetic complexes of the type [F3NMe]W(CH2R)(CR), where R ) CMe3, SiMe3 respectively. X-ray studies confirmed the structures of [F3NMe]Mo(CH2CMe3)(CCMe3) and {[F3NMe]Mo(CSiMe3)}2. The chemistry of complexes that contain a triamidoamine ligand has been under investigation in our laboratories for the past few years.1-6 Although the focus has been on dinitrogen reduction and functionalization by Mo complexes,7-10 we also had the opportunity to prepare and explore Mo(IV) or W(IV) alkyl complexes.2,3,6 In the process we discovered an unusual R,R-dehydrogenation reaction that converts a Mo(IV) or W(IV) alkyl complex into an alkylidyne complex and molecular hydrogen. This reaction is much more facile for W than for Mo and in many cases takes place even when β-hydride elimination or abstraction processes would be plausible alternatives. We have turned to the synthesis and study of diamidoamine complexes of Mo and W, initially those that contain the [(C6F5NCH2CH2)2NMe]2- ligand,11 primarily in an effort to prepare dinitrogen complexes in which the dinitrogen is more sterically accessible and therefore could be more sus(1) Schrock, R. R. Acc. Chem. Res. 1997, 30, 9. (2) Schrock, R. R.; Seidel, S. W.; Mosch-Zanetti, N. C.; Shih, K.-Y.; O’Donoghue, M. B.; Davis, W. M.; Reiff, W. M. J. Am. Chem. Soc. 1997, 119, 11876. (3) Seidel, S. W.; Schrock, R. R.; Davis, W. M. Organometallics 1998, 17, 1058. (4) Greco, G. E.; O’Donoghue, M. B.; Seidel, S. W.; Davis, W. M.; Schrock, R. R. Organometallics 2000, 19, 1132. (5) Reid, S. M.; Neuner, B.; Schrock, R. R.; Davis, W. M. Organometallics 1998, 17, 4077. (6) Schrock, R. R.; Seidel, S. W.; Mo¨sch-Zanetti, N. C.; Dobbs, D. A.; Shih, K.-Y.; Davis, W. M. Organometallics 1997, 16, 5195. (7) Kol, M.; Schrock, R. R.; Kempe, R.; Davis, W. M. J. Am. Chem. Soc. 1994, 116, 4382. (8) O’Donoghue, M. B.; Davis, W. M.; Schrock, R. R. Inorg. Chem. 1998, 37, 5149. (9) O’Donoghue, M. B.; Davis, W. M.; Schrock, R. R.; Reiff, W. M. Inorg. Chem. 1999, 38, 243. (10) Greco, G. E.; Popa, A. I.; Schrock, R. R. Organometallics 1998, 17, 5591. (11) Cochran, F. V.; Bonitatebus, P. J.; Schrock, R. R. Organometallics 2000, 19, 2414.
ceptible to new modes of functionalization. Recent evidence that this is the case for V and Ta consists of reports that describe reductive cleavage of dinitrogen between two vanadium centers12 and the reaction between dinitrogen and a tantalum hydride to give a bridging dinitrogen unit that is bound both side-on and end-on.13 The fact that the reaction between (C6F5NHCH2CH2)2NMe and Mo(NMe2)4 led to seven-coordinate [(C6F4(NMe2)NCH2CH2)2NMe]MoF2 rather than five-coordinate [(C6F5NCH2CH2)2NMe]Mo(NMe2)2 suggested to us that the chemistry of complexes that contain the [(C6F5NCH2CH2)2NMe]2- ligand ultimately might be limited by undesirable nucleophilic attack on the pentafluorophenyl ring.11 Here we report the synthesis and characterization of Mo and W complexes that contain the apparently more robust [(3,4,5-C6H2F3NCH2CH2)2NMe]2- ligand and some reactions that suggest that R,R-dehydrogenation reactions can be found in environments outside of the triamidoamine platform. In a recent publication14 the transformation of pentafluorophenyl rings bound to nitrogen into what were proposed to be 2,4,6-C6H2F3 rings with LiAlH4 in refluxing THF was reported.15 In the belief that 2,4,6C6H2F3 rings might be less susceptible to nucleophilic attack, we attempted a similar reaction between (C6F5NHCH2CH2)2NMe and LiAlH4.16 We were gratified to find that (C6H2F3NHCH2CH2)2NMe was indeed formed in good yield but surprised to learn through X-ray studies of compounds described below that the product was (3,4,5-C6H2F3NHCH2CH2)2NMe, not (2,4,6-C6H2F3NHCH2CH2)2NMe (eq 1). This outcome may be the more 4.5LiAlH4
(C6F5NHCH2CH2)2NMe 9 8 THF, reflux, 4 h (3,4,5-C6H2F3NHCH2CH2)2NMe (H2[F3NMe]) (1) desirable of the two, as the reaction between H2[F3NMe] and Mo(NMe2)4 was found to yield stable five-coordinate diamagnetic [F3NMe]Mo(NMe2)2 as a red-brown powder, (12) Clentsmith, G. K. B.; Bates, V. M.; Hitchcock, P. B.; Cloke, F. G. N. J. Am. Chem. Soc. 1999, 121, 10444. (13) Fryzuk, M. D.; Johnson, S. A.; Rettig, S. J. J. Am. Chem. Soc. 1998, 120, 11024. (14) Buchler, S.; Meyer, F.; Jacobi, A.; Kircher, P.; Zsolnai, L. Z. Naturforsch. 1999, 54b, 1295. (15) We thank S. Buchler for bringing this publication to our attention. (16) A solution of (C6F5NHCH2CH2)2NMe (3.37 g, 7.49 mmol) in THF (20 mL) was added via syringe to LiAlH4 (1.28 g, 33.74 mmol) in THF (30 mL) at room temperature. The reaction mixture was heated to reflux for 4 h. The reaction was quenched with water, and all volatile components were removed in vacuo. The residue was washed with 0.5 M NaOH (200 mL) and extracted twice into 200 mL of dichloromethane. The product was isolated as a thick yellow oil from the organic layers; yield 2.46 g (87%).
10.1021/om0100176 CCC: $20.00 © 2001 American Chemical Society Publication on Web 04/27/2001
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Communications
in contrast to activation of the ortho fluorides in (C6F5NHCH2CH2)2NMe upon reaction with Mo(NMe2)4.11 The reaction between MoCl4(THF)2, H2[F3NMe], and NEt3 in THF led to a paramagnetic purple product formulated as [Et3NH]{[F3NMe]MoCl3} on the basis of 19F and 1H NMR spectra and elemental analysis (eq 2). 2.2 NEt3,THF
H2[F3NMe] + MoCl4(THF)2 9 8 22 °C, 1 h [Et3NH]{[F3NMe]MoCl3} (2) This reaction is analogous to the synthesis of [Et3NH]{[(C6F5NCH2CH2)2NMe]MoCl3};11 it proceeds in good yield despite the fact that there are only three fluorides on each of the ligand aryl rings. Cation exchange with Bu4NCl produced a more soluble tetrabutylammonium salt. The magnetic moment of [Bu4N]{[F3NMe]MoCl3} was determined by the Evans method17 as modified by Sur18 to be 3.1 µB, slightly higher than the spin-only value for two unpaired electrons (2.83 µB). A reaction similar to that shown in eq 2 between WCl4(dme), H2[F3NMe], and NEt3 in diethyl ether produced [Et3NH]{[F3NMe]WCl3}. A tetrabutylammonium salt was prepared in the same manner as described above for [Bu4N]{[F3NMe]MoCl3}. A measurement of the magnetic moment of [Bu4N]{[F3NMe]WCl3} by the Evans method gave µeff ) 2.7 µB. Addition of 3 equiv of Me3CCH2MgCl to [Et3NH]{[F3NMe]MoCl3} in THF yielded [F3NMe]Mo(CH2CMe3)(CCMe3) as an amber crystalline solid in 68% yield.19 The alkylidyne carbon resonance was located at 308 ppm in the 13C NMR spectrum. An X-ray diffraction study revealed the complex to be approximately a trigonal bipyramid with the neopentylidyne ligand occupying an axial position trans to the amine donor (Figure 1). All bond lengths and angles are in the expected range. Note that the aryl rings contain fluorides in the 3-, 4-, and 5-positions. We hypothesize that this species arises via R,R-dehydrogenation of transient [F3NMe]Mo(CH2CMe3)2 (eq 3). The ease of R,R-dehydrogenation in
[F3NMe]Mo(CH2CMe3)2 stands in contrast to that process in isolable Mo(IV) neopentyl complexes containing the [(Me3SiNCH2CH2)3N]3- and [(C6F5NCH2CH2)3N]3ligands, which are converted to the corresponding neopentylidyne complexes at appreciable rates only when heated.2,3 An unusual feature of R,R-dehydrogenation in [F3NMe]Mo(CH2CMe3)2 is that dihydrogen (17) Evans, D. F. J. Chem. Soc. 1959, 2003. (18) Sur, S. K. J. Magn. Reson. 1989, 82, 169. (19) A solution of NpMgCl (0.43 mL, 2.27 M in Et2O) was added to a solution of [Et3NH]{[F3NMe]MoCl3} (0.2 g, 0.29 mmol) in THF (10 mL) at -30 °C. After the reaction mixture was warmed to room temperature and stirred for 1 h, all volatile components were removed in vacuo. The residue was extracted into toluene (20 mL) and filtered through Celite to remove magnesium salts. The filtrate was added dropwise to a rapidly stirred volume of cold pentane (100 mL) and filtered a second time. A brown crystalline solid was isolated upon concentrating the solution to dryness; yield 0.122 g (68%). Analytically pure, amber crystals suitable for X-ray diffraction were grown from a mixture of benzene and pentane at -30 °C.
Figure 1. Thermal ellipsoid plot (30% probability level) of the structure of [F3NMe]Mo(CH2CMe3)(CCMe3). Selected bond distances (Å) and angles (deg): Mo-C(1) ) 1.776(3), Mo-C(6) ) 2.143(3), Mo-N(1) ) 2.042(2), Mo-N(2) ) 2.392(2); N(1)-Mo-N(3) ) 126.43(9), C(6)-Mo-N(1) ) 112.67(11), C(1)-Mo-N(2) ) 170.66(10), C(2)-C(1)-Mo ) 173.0(2), C(7)-C(6)-Mo ) 129.7(2). Crystal data: C27H35F6N3Mo, monoclinic, P21/n, Z ) 4, a ) 12.5977(9) Å, b ) 14.7520(11) Å, c ) 15.0992(11) Å, β ) 98.1530(10)°, R1 ) 0.0294, wR2 ) 0.0812 (all data).
rather than neopentane is formed, although the immediate product upon loss of neopentane would be what is perhaps a relatively disfavored Mo(IV) neopentylidene complex. A minor byproduct isolated from this alkylation reaction (
